Apparatus for use in determining a characteristic of a cathode ray tube

ABSTRACT

Test apparatus for a cathode ray tube has optical elements disposed along two optical paths from the screen of a cathode ray tube to two photosensors. One optical path passes through a graticule which has a pattern of light transmissive and non-transmissive areas. The real image of a luminous spot on the screen is accurately positioned on a boundary between two of the areas of the graticule by deriving a control signal from a comparison of the photosensor outputs.

This invention relates to apparatus for use in determining thecharacteristic of a cathode ray tube relating luminous spot position todeflection current.

Conventionally the aforesaid characteristic is determined by positioningthe spot in a number of places over the tube face and measuring the spotposition by a travelling microscope. The current flowing in thedeflection windings of the cathode ray tube is recorded for each spotposition so identified; and detailed analysis of the matrix of valuesrelating spot position with current value enables the tube quality to beassessed.

The aforesaid method suffers from certain disadvantages. Firstly, themethod is slow and estimation of the spot center requires a subjectivedecision which may be difficult where the spot is not of uniformbrightness distributiion. Secondly, estimation of spot width is alsosubjective and is made more difficult by non-uniform spotcharacteristics.

Accordingly the present invention, provides apparatus for use inascertaining the characteristic of a cathode ray tube relating luminousspot position to deflection current comprising:

A LOCATING MEANS FOR A CATHODE RAY TUBE; OPTICAL MEANS WHICH DEFINE AFIRST OPTICAL PATH BETWEEN THE SCREEN OF A CATHODE RAY TUBE LOCATED BYSAID LOCATING MEANS AND A FIRST PHOTO-SENSOR; A GRATICULE DISPOSED INSAID FIRST OPTICAL PATH, SAID GRATICULE CONTAINING A PREDETERMINEDSPATIAL DISTRIBUTION OF LIGHT TRANSMISSIVE AND NON-TRANSMISSIVE AREAS,THE FIRST OPTICAL PATH INCLUDING IMAGING MEANS OPERATIVE TO FORM A REALIMAGE OF A LUMINOUS SPOT ON SAID CATHODE RAY TUBE SCREEN AT ACORRESPONDING POSITION ON SAID GRATICULE; FURTHER OPTICAL MEANS WHICHDEFINE A SECOND OPTICAL PATH BETWEEN THE SCREEN OF THE CATHODE RAY TUBEAND A SECOND PHOTO-SENSOR, A FIRST SIGNAL COMPARATOR RESPONSIVE TO THEOUTPUTS FROM SAID FIRST AND SECOND PHOTO-SENSORS; MEANS RESPONSIVE TOTHE OUTPUT OF SAID FIRST COMPARATOR FOR PRODUCING A SIGNAL FORAPPLICATION TO A BEAM DEFLECTION MEANS OF THE CATHODE RAY TUBE TOPOSITION SAID IMAGE OF THE SPOT ON THE GRATICULE SUCH THAT THE OUTPUTSPRODUCED BY SAID FIRST AND SECOND PHOTO-SENSORS ARE IN A PREDETERMINEDRELATIONSHIP.

Preferably the apparatus further comprises means for causing said spoton the screen of the cathode ray tube and hence said image of the spoton the graticule to move to different positions in which the outputsproduced by said first and second photo-sensors are in saidpredetermined relationship.

Preferably said means for causing said spot on the screen of the cathoderay tube to move to different positions comprises a cyclic rampgenerator for supplying or controlling the current to the beamdeflection means so as to deflect the luminous spot in a direction atright angles to the direction in which the spot moves in response to thephoto-sensor outputs, and a comparator which is connected at its inputto the output of said cyclic ramp generator and is adapted to receive atanother input programmed positioning signals;

and wherein at equality between the ramp waveform and a programmedsignal said comparator develops an output for use in enabling bright-upcircuitry of the cathode ray tube.

One apparatus in accordance with the invention is hereinafter describedby way of example with reference to the accompanying drawings in which:

FIG. 1 is a block schematic diagram of the apparatus; and,

FIG. 2 shows an enlarged portion of a graticule used in the apparatus ofFIG. 1.

Referring to FIG. 1 the apparatus comprises a photosensor 11, a fieldlens 39, a graticule 25 and an objective lens 37 which lie along anoptical path O₁ extending from the screen 33 of a cathode ray tube 29.The cathode ray tube 29 is shown in FIG. 1 located at the appropriateposition and supported by a locating and supporting arrangement 9 whichforms part of the apparatus. The apparatus further comprises a secondphotosensor 15 lying along a second optical path O₂ from the screen 33of the cathode ray tube, and a signal comparator 17 connected to theoutputs of the photosensors 11 and 15. Deflection control circuitry 19is connected to the output of the comparator 17 by way of a reversingswitch 21.

Referring to FIG. 2, the graticule 13 contains a predetermined spatialdistribution of light transmissive and non-transmissive areas 23 and 25respectively. As shown the graticule 13 has a chequered pattern ofalternating light transmissive areas; but it should be understood thatother distributions may be utilized. Indeed, provided that the positionsof the transmissive and non-transmissive areas are known thedistribution may take any suitable form.

The deflection control circuitry 19 in response to an output from thecomparator 17 develops an output signal for producing or controlling theproduction of a current in deflection windings 27a for deflecting theelectron beam of a cathode ray tube 29 when such a tube is supported bythe locating means 9.

With the cathode ray tube 29 energized so as to produce a luminous spot,as 31, at the screen 33 thereof a real image of the spot 31 is producedat the graticule 13. At least a fraction of any light transmittedthrough the graticule 13 (as a result of the real image 35 (in FIG. 2)or a part thereof being coincident with a light transmissive area 23) isincident on the photosensor 11.

As may be seen the real image 35 is produced from the spot 31 by theobjective lens 37. The field lens 39 directs light transmitted by thegraticule 13 to the photosensor 11.

It will be appreciated that production of the real image 35 may beachieved by other means. For example a zone plate, though less efficientas a focussing device, might be employed provided that the photosensor11 is of adequate sensitivity. Further it might be possible to dispenseentirely with the field lens 39 by providing a graticule 13 which has acoating of photosensitive substance adherent thereto.

Light propagated along the second optical path O₂ passes through anaperture 43 of a member 41. The member 41 is in the form of an iris soas to permit adjustment of the size of the aperture 43. Intercepting theoptical path O₂ is an objective lens 45 and a field lens 47. As with theoptical path O₁ other optical arrangements are possible.

The difference signal from the comparator 17, in response to the outputsof the photosensors 11 and 15 is, after filtering by deflection controlcircuitry 19, applied to the deflection windings 27a.

The transfer function of the deflection control circuitry 19 is suchthat when a real image 35 of the luminous spot is present on thegraticule the current in the windings 27a varies until a predeterminedfraction only of the real image 35 is coincident with a lighttransmissive area 23, the remainder of the image then being coincidentwith the contiguous non-transmissive area 25 of the graticule 13. Theaforesaid transfer function has, essentially, a proportional componentand a pseudo-integral or low band-pass component to ensure that the realimage 35 is driven so as to reduce the difference signal from thecomparator 17 to zero within fine limits of accuracy, thereby ensuringaccurate positioning of the real image 35 with respect to a particularboundary between transmissive and non-transmissive areas of thegraticule.

In practice it is desirable that the predetermined fraction of the realimage 35 which is coincident with the light transmissive area is suchthat one half of the total light flux from the real image is transmittedthrough the light transmissive area. Accordingly, prior to commencingthe calibration of a cathode ray tube, with the spot 31 so positionedthat the total area of the real image is coincident with a lighttransmissive area 23 of the graticule 13, the member 41 is adjusted sothat the aperture 43 is of a size such that the output from thephotosensor 15 is equal to one half of that from the photosensor 11.

With the apparatus so far described, a simple calibration programme maybe carried out. A fixed, known current is passed under the control of acontrol circuit 61 through the deflection windings 27b giving theluminous spot 31 a position in the x-direction such that its real imageon the graticule lies in, say, column A of the graticule pattern shownin FIG. 2. The current in deflection windings 27a is then subjected toincremental changes by the control circuit 61 so that the image of theluminous spot moves successively to light transmissive areas in they-direction along column A. Once incident on a light transmissive area,say that bounded by horizontal boundaries 2 and 3 of FIG. 2, the imageof the luminous spot is move by the deflection control circuitry 19controlled by the output of comparator 17 so that it is accuratelyplaced on the boundary 2 or 3 as described above. The direction that thespot moves is determined by the reversing switch 21, so that if theimage initially aligns on the boundary 2 in column A, the reversingswitch 21 is then reversed so that the image aligns with boundary 3. Inthis way two y-direction current values are obtained for eachpositioning of an image in a light transmissive area. The process isrepeated in the other columns, and then the deflection windingsconnections are interchanged and the whole process repeated to givex-direction current values for fixed y values.

The addition of further apparatus allows a more sophisticatedcalibration programme to be carried out. This apparatus comprises astrobe ramp generator 49 and a programmed spot position signal generator51 (which may constitute, with the control circuit 61, part of a digitalcomputer). The strobe ramp generator 51 is shown connected to thecathode ray tube deflection windings 27band thus controls movement ofthe luminous spot in the x-direction. The outputs from generator 49 andgenerator 51 provide the inputs to a comparator 53 whose output isconnected to the bright-up circuitry of the cathode ray tube.

Upon commencement of the programme the bright-up circuitry is notenabled and the deflection of a spot is purely notional. During theperiod of a strobe signal the notional spot undergoes deflection in thex-direction, and when the amplitude of this strobe signal equals theamplitude of one of a number of positioning signals of differingamplitude supplied by the generator 51 under the control of the controlcircuit 61, the comparator 53 responds by developing a pulse whichenables the bright up circuitry of the cathode ray tube.

The programmed positioning signals are predetermined by approximatecomputation so as to correspond to those values of the x-directiondeflection current, i.e., in windings 27b, that produce an image of theluminous spot incident on a vertical boundary of the graticule. Thus anenabling pulse is produced by comparator 53 and a spot appears on thescreen at each point along the path of the notional spot parallel to thex-axis when that path crosses a vertical boundary.

The enabling pulse for the comparator 53 must have a duration shortenough to prevent the spot being present on the screen so long that itsimage is incident with the next vertical boundary of the graticule, butit must be long enough to ensure that the deflection control circuitry19 has sufficient time to position the spot so that its image isaccurately positioned, i.e., at 50% cut-off, with respect to theappropriate portion of the horizontal boundary.

By way of example, the procedure will be described with reference toFIG. 2. Initially the current in deflection windings 27a isapproximately adjusted such that the spot, when present, would have itsimage adjacent the boundary 2. This adjustment is made in order todecrease the movement required to be produced by the deflection controlcircuitry 19. When the deflection produced by the strobe signal is suchthat the image of the spot, if present, would be crossing the verticalboundary entering into column A, the spot is brightened up, and thenunder the control of the deflection control circuitry is accuratelypositioned on the portion of boundary 2 that is in column A. The spot isthen extinguished and brightened-up again as it enters column B. Thereversing switch 21 is controlled by the control circuit 61 so as toreverse at successive boundaries and so to move the spot in the requireddirection in each column.

In this way a series of current values for deflection in the y-directionare obtained for various positions of the image of the spot along astraight line on the graticule and correspondingly on the screen.

The procedure is repeated for horizontal boundaries 3, 4, etc., and isthen completely repeated with the deflection windings interchanged, togive current values for deflection in the x-direction. From theseresults a complete matrix relating spot position to deflection currentcan be defined.

After sampling the deflection current values corresponding to thevarious spot positions so as to derive digital representation of thesaid current values the spot position/deflection current matrix may,using a digital computer, on-line or otherwise, be analysed and outputdata provided indicating the tube spot position/deflection currentcharacteristic or indicating on the basis of a test criterion or testcriteria, including in the computer program, whether or not the cathoderay tube is acceptable.

It should be understood that the deflection windings 27a and 27b may besupplied as component elements of the cathode ray tube; or they may bestandard windings which form part of the apparatus and which are adaptedto receive a tube devoid of its own deflection windings.

We claim:
 1. Apparatus for use in ascertaining the characteristic of acathode ray tube relating luminous spot position to deflection current,said apparatus comprising:(A) a locating means for a cathode ray tube;(B) a first photosensor; (C) optical means defining a first optical pathbetween the screen of a cathode ray tube located by said locating meansand the first photosensor; (D) a graticule disposed in said firstoptical path,(i) said graticule containing a predetermined spatialdistribution of light transmissive and non-transmissive areas, (ii) thefirst optical path including imaging means operative to form a realimage of a luminous spot on said cathode ray tube screen at acorresponding position on said graticule so that the output of saidfirst photosensor has a value dependent on the position of said spot onthe cathode ray tube screen; (E) a second photosensor; (F) furtheroptical means which defines a second optical path between the screen ofthe cathode ray tube and the second photosensor,(i) said second opticalpath having a predetermind constant light transmissivity so that theoutput of said second photosensor has a constant value independent ofthe position of said spot on said cathode ray tube screen; (G) a firstsignal comparator responsive to the outputs from said first and secondphotosensors; and (H) means responsive to the output of said firstcomparator for producing a signal for application to a beam deflectionmeans of the cathode ray tube to position said spot and hence said imageof the spot on the graticule such that the outputs produced by saidfirst and second photosensors are in a predetermined relationship. 2.Apparatus as claimed in claim 1 which further comprises means forcausing said spot on the screen of the cathode ray tube and hence saidimage of the spot on the graticule to move the different positions inwhich the outputs produced by said first and second photosensors are insaid predetermined relationship.
 3. Apparatus as claimed in claim 2wherein said means for causing said spot on the screen of the cathoderay tube to move to different positions comprises a cyclic rampgenerator for supplying or controlling the current to the beamdeflection means so as to deflect the luminous spot in a direction atright angles to the direction in which the spot moves in response to thephotosensor outputs, and a comparator which is connected at its input tothe output of said cyclic ramp generator and is adapted to receive atanother input programmed positioning signals; and wherein at equalitybetween the ramp waveform and a programmed signal said comparatordevelops an output for use in enabling bright-up circuitry of thecathode ray tube.
 4. Apparatus according to claim 1 wherein saidpredetermined spatial distribution of light transmissive andnon-transmissive areas contained in the graticule is in the form of achequered pattern of alternating light transmissive and non-transmissiveareas.
 5. Apparatus as claimed in claim 1 wherein means are provided forsetting the light transmissivity of said second optical path, such thatwhen said image of the luminous spot falls wholly on a lighttransmissive area of the graticule then the output produced by thesecond photosensor is half that produced by the first photosensor. 6.Apparatus as claimed in claim 5 wherein said means provided for settingthe light transmissivity of said second optical path comprises avariable aperture diaphragm disposed in said second optical path. 7.Apparatus as claimed in claim 6 wherein said predetermined relationshipbetween the outputs produced by said first and second photosensors isthe output produced by the first photosensor is equal to the outputproduced by the second photosensor.